Astringent compound (6)
(Other definitions for tannin that I've seen before include "Substance in tea, wine, etc" , "It's found in tea and wine" , "Compound used to convert hide into leather" , "Something in red wines" , "It may be found in red wines" .)
'astringent compound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alum that I've seen before include "Name given to a specific chemical compound and a class of them" , "White mineral salt used in medicine and in dyeing" , "double sulphate" , "Compound used in tanning and dying" , "Colourless compound used in dyeing and tanning" .)